This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This research tests a simple, plausible, and novel proposal about how the cerebellum makes voluntary rapid eye movements (saccades) accurate. We propose that saccade accuracy depends on a signal that stops saccades at the right time. This signal travels across the midline of the cerebellum in the axons called parallel fibers (p-fibers). The onset of saccade deceleration is set by the amount of time it takes the signal to travel to its destination on the other side. We will test this proposal by cutting saccade-related p-fibers. If this idea is right, then this cut will: 1) abolish the cerebellum signal that stops saccades on time; 2) make saccade deceleration abnormally slow;3) make saccades overshoot their targets. Preliminary data show that this cut causes all three effects. A corollary of our proposal is that large saccades are represented laterally in the saccade-related part of the cerebellum while small saccades are represented medially. We will use anatomical tracing to describe the connections from lateral and medial parts of this region to determine how these connections cause movements of different sizes. Preliminary data show that the lateral and medial areas of the saccade part of the cerebellum make different connections. Describing this will tell us how the cerebellum makes movements of different sizes. If our results support our proposal, then it will provide a basic framework for how the cerebellum transforms the saccade-related signals that it receives into the output signals that make saccades accurate. This work will also show why saccades, and consequently vision, are badly impaired by surgery to remove the most common brain tumors in children. This will motivate and guide easy-to-implement modifications of treatment and surgery to removes tumors without impairing vision. Such improvements could help patients quickly because they require no other innovations.